This invention relates generally to continuous rotary heat-treating furnaces and more particularly, it relates to a rotary retort furnace used for continuous heat processing of workpieces which include a retort assembly formed of a cylindrical axial member and a spirally-wound conduit disposed on the exterior surface of the axial member to move the workpiece from one end of the retort assembly to the other end.
Rotary retort furnaces have been utilized for many years for the continuous treatment of a variety of small metal workpieces or parts, such as bolts, nuts, screws, rivets, pins, balls, springs, clips, studs, nails, washers and the like. In these prior art furnaces, the retorts are used for the continuous processing of small packets of such parts and coneys them from one end of the retort to the other end as it rotates within a surrounding heating chamber. Such prior retorts are typically cylindrical in shape which have been generally either heavy walled rough cast retorts with internal cast spirals (flights) or fabricated retorts of wrought materials with the internal spiral welded directly to the cylindrical shell. The fabricated retorts of wrought materials have several inherent problems. One of the problems is that the manufacture of the internal flight is very expensive due to the high tolerance requirement in locating and attaching of the internal flights in close contact relationship with the sidewalls of the retort. Otherwise, the processed parts will tend to become lodged between the flight and the sidewalls. If such parts to be treated in one operation become lodged in the spaces between the flights and sidewalls of the retort and subsequently fall into or become mixed with a different sets of parts being heat treated, it can prove to be quite a burdensome and expensive task to sort the parts. The mixing of parts is not acceptable. If the amount of dwell time is controlled to be very precise, then the mixing of the parts would either prolong or reduce the dwell time. Another problem of the fabricated retorts of wrought materials is that the strength of the materials used cannot readily withstand the cyclic physical and thermal stresses which can inherent in the rotary furnaces at high temperatures.
While the cast retorts are much better able to cope with the high temperature stresses, they are not in any way free of problems. For example, since the wall of cast retorts is much thicker than in the wrought retorts, a much higher thermal environment is required to compensate for the thermal gradient in the retort wall in order to achieve the same internal temperatures. Further, due to the casting process the internal surfaces are considerably rougher than the wrought retorts which will cause a buildup of clinker (cutting swarf and cutting oils) restricting the passage of the parts and cause a reduction in the heat exchange rates and may possible damage the parts themselves.
Since rotary retort furnaces are often used with a controlled atmosphere to create a neutral (non-oxidizing) condition or to create a thermochemical combination required for heat treatment of parts, it is important to minimize the amount of gas or gases used within the furnace. In prior art designs of continuous rotating furnaces, considerable volumes of gas are utilized due to the retort configuration and the location of the small packets of parts so as to assure that a sufficient quantity of controlled atmosphere is applied to all the parts and a sufficient time is provided to obtain the desired treatment. However, the volume of gases used are relatively high, thereby increasing operating cost.
Another problem associated with these prior arts rotary furnaces is the thermal stresses developed in the internal flight and the shell of the retort due to the fact that the workpieces enter the furnaces in an extremely cold state relative to the temperature of the furnace. The degree of stress depends upon the magnitude of the temperature differential. This temperature difference can be quite substantial since the heat inputs applied to the furnace entrance is very high. As a result, the presence of this and other typical conditions may cause premature failure of the retort by cracking occurring adjacent the intersection of the flights and the shell, thereby requiring costly replacement or repair of flight and/or retort shell. Still another deficiency which has been traditionally encountered in these retort furnaces heretofore is that heat transferred to the parts was limited primarily to radiation with some heat transfer by conduction to parts adjacent to the sidewalls of the retort.